The applicant will continue to describe the topography and function of the several receptor sites presented by systems mediating membrane transport, and the response these sites make on occupation by a substrate and a co-substrate. The interaction between various substrates and alternative co-substrates will also be studied. For these purposes we select and design amino acid analogs with restricted or exaggerated responses to the forces inherent in uphill transport. Amino acids with modified dissociation behavior are used to investigate how these are accommodated by transport systems serving for ordinary amino acids. We plan to extend the studies to primary cultures of hepatocytes, and to fluorescent amino acid analogs, as well as continuing with the Ehrlich ascites tumor cell and nucleated red blood cells. BIBLIOGRAPHIC REFERENCES: Christensen, H.N., 1976, Metabolite transport at the cell membrane, in Transport Phenomena in the Nervous System, G. Levi, L. Battistin, and A. Lajtha, Eds., Plenum, N.Y., 3-12. Im, W.B., Christensen, H.N., and Sportes, B., 1976, Amino acid stimulation of ATP cleavage by two Ehrlich cell membrane preparations in the presence of onabain, Biochem. Biophys, Acta 436, 424-437.